Conventionally, an antenna represented by a planar antenna with perturbation has characteristics in having a narrow axial ratio band and maintaining a satisfactory axial ratio near the designed frequency, but in that the axial ratio characteristics significantly degrades when the frequency shifts. This state is shown in FIGS. 15(a) and 15(b), where FIG. 15(a) is a graph showing the axial ratio characteristics, and FIG. 15(b) shows a polarization state at the respective frequency. As apparent from the graph, the axial ratio is substantially 0 dB and is satisfactory at the designed frequency, that is, near the center frequency f0, but the axial ratio characteristic significantly degrades at f−, which is shifted to the − side, and at f+, which is shifted to the + side, with respect to the center frequency. In the polarization state, circular polarization is obtained at the center frequency f0, but an elliptical polarization inclined to the left or the right is obtained and the axial ratio is significantly degraded at f− and f+.
A sequential array antenna in which planar antennas with perturbation are sequentially arranged has been developed in recent years (see e.g., paragraph 0027 of Patent Document 1). The sequential array antenna is arranged with a plurality of antenna elements, and is excited with each antenna element rotated by 180/n (n=1, 2, 3, . . . ) and the phase also changed by 180/n (n=1, 2, 3, . . . ). For instance, as shown in FIG. 16, when the sequential array antenna is configured by linearly arranging three antenna elements, each having one power supply point and opposing cutouts (perturbation), each antenna element is arranged after being mechanically rotated according to the following equation φn=(n−1)π/N (n: nth antenna element, N: number of antenna elements, N=3 in the case of three antenna elements).
In the sequential array antenna including N elements, a complete circular polarization is radiated irrespective of the polarization of the antenna elements in the broadside direction (direction orthogonal to the arranging direction of the antenna elements) when the rotation of equation φn=(n−1)π/N and phase deviation are applied to the nth antenna element, so that satisfactory circular polarization and impedance characteristics can be maintained over a wide band.
However, when using a frequency (communication channel) shifted from the center frequency, the directional characteristics of the sequential array antenna become as shown in FIGS. 17(a) to 17(d) and a problem in that the directional characteristics change by the frequency arises. In particular, when controlling the directional direction as a phased array antenna in combination with a phase shifter, the beam direction changes by the frequency. This is particularly significant when the communication counterpart is a linear polarization as in RFID, and the reception area tends to change by the frequency. FIGS. 17(a) to 17(d) show the directional characteristics and the axial ratio characteristics of the sequential array antenna, where FIGS. 17(a) and 17(b) show the state of the beam when the frequency f+ is used, and FIGS. 17(c) and 17(d) show the state of the beam when the frequency f− is used. Here, Eθ is the horizontal component of the circular polarization and Eφ is the vertical component, where in the cases of frequency f+ and frequency f−, the beam direction is left and right opposite although the gain does not change and the axial ratio characteristics do not change in Eθ and Eφ, and furthermore, change exists in Eθ and Eφ when beam shifted in combination with the phase shifter, as shown in FIGS. 17(b) and 17(d).
In a case of a general phased array antenna in which antenna elements with perturbation having the same antenna direction are linearly arranged as shown in FIG. 18, the directional characteristics do not depend on the frequency but fluctuation in gain becomes large as shown in FIGS. 19(a) to 19(d). FIGS. 19(a) to 19(d) show the directional characteristics of the phased array antenna, where FIGS. 19(a) and 19(b) show the state of the beam when the frequency f+ is used, and FIGS. 19(c) and 19(d) show the state of the beam when the frequency f− is used. In the cases of frequency f+ and frequency f−, the gain is opposite although the front direction is being faced and change is not found in the directional characteristics in both Eθ and Eφ. Similar to the above, change exists in Eθ and Eφ when beam shifted.
In other words, if the sequential array antenna or the phased array antenna is configured using a planar antenna element in which the individual antenna axial ratio band is low, the broadside direction maintains satisfactory axial ratio characteristics over a wide band regardless of the change in frequency but the directional direction fluctuates due to change in frequency in the sequential array antenna. In the phased array antenna, the directional direction does not fluctuate due to change in frequency, but the axial ratio fluctuates due to change in frequency. Thus, the respective array antennas have advantages and disadvantages in the directional characteristics and the axial ratio band.
The following method is known as a method for solving the problems of the background art. One method of improving the axial ratio band is a method of thickening the thickness of the substrate that configures the array antenna or lowering the substrate dielectric constant. However, the use of such a method arises other problems in that the size of the antenna becomes large and miniaturization cannot be achieved, the manufacturing cost increases, and the like. Another method of improving the axial ratio band is a method of providing the power supply point at two regions, but such a method also arises a different problem in that the power supply circuit becomes complicating. In addition, a method of increasing the antenna element not only in the horizontal row but also in the vertical row in the sequential array antenna to obtain a so-called sequential sub-array configuration is known, but such a method also arises a different problem in that the size of the antenna becomes large. Therefore, if the above-described problems are solved with the methods of the background art, problems such as enlargement of the antenna size and complication arise, and a satisfying method for solving is not yet proposed.
Patent Document 1: Japanese Unexamined Patent Publication No. 09-98016